Perovskite solar cells have received great attention lately due to their unique benefits such as high efficiency and low cost achieved in short time as well as its easiness to fabricate (green et. al. 2014). Perovskite materials have been recently studied as photo anode, especially the methylammonium lead iodide (MAPI). Organometallic halide perovskite solar cells have extremely high-power conversion efficiencies of 22.1%, as officially recognized by the US National Renewable Energy Laboratory (NREL) (Yang et. al. 2017). These solar cells are therefore attractive candidates for use in next generation photovoltaic applications. The methylammonium lead iodide with anatase-TiO2 is a good candidate for perovskite solar cell due to their similarity in band structure as a n-type semiconductor. The energy of the MAPI band gap is similar or slightly higher than anatase-TiO2, which is proven to enhance the open circuit voltage (VOC) of a perovskite solar cell (O'Regan, Brian C et. al. 2015). The enhancement of TiO2-MAPI combination has been reported previously in literature, for example, Suzuki and Okamoto made double-layer perovskite solar cells they studied the order effect of the TiO2-perovskite combination (Okamoto, Yuji, and Yoshikazu Suzuki. 2016). The highest efficiency reached was 5.71% by applying TiO2 as the first layer and perovskite as the second layer. They explained the enhancement by that perovskite as second layer has increased the open circuit voltage VOC of the solar cell, which will decrease the electron-hole recombination and thus increase the efficiency. While several issues remain to be addressed regarding device operation and stability, the incorporation of carbon-based nanostructures was rapidly proposed, and significant advances have been achieved so far (Xiao, Junyan, et al. 2015).
Carbon-based nanomaterials have led to extensive research efforts in the field of photovoltaic energy conversion in the last two decades. Initially exploiting carbon nanotubes, carbon-based solar cells have now largely taken benefit from graphene and its various forms to demonstrate significant improvements in almost all device functions including charge generation, charge collection, and charge transport (Habisreutinger, Severin N., et al. 2014). More recently, hybrid organometallic halide perovskite became one of the most promising materials for third generation solar cells, with efficiencies now competing with thin film technologies (Ku, Zhiliang, et al. 2013). Two-dimensional (2D) materials have been proven to increase the efficiency of solar cells specially graphene due to its unique electronic properties. A work done by Di Carlo et al. where they prepared a solar cell by using mesoporous layer TiO2 doped with graphene flakes (mTiO2+G), also they used an interlayer made from graphene oxide between the hole-transport layer and the perovskite. They achieved 18.2% power conversion efficiency and they owed the enhancement to the improvement of charge-carrier mobility (injection/collection) process. In addition, they found that 2D material has increased the stability; they tested it under one sun illumination after 16 hours and found it more stable when it is doped with graphene flakes. Moreover, at 60° C., they found that the interlayer GO has increased the stability even more (Agresti, Antonio, et al. 2016). In another work, Robin J. Nicholas and co-workers reported the use of a nanocomposite made of TiO2 doped graphene flakes as electron collection meso-porous layer in perovskite solar cells. They also found that the nano flakes of graphene doped in TiO2 has increased the charge collection layer in the nanocomposites, which lead them to achieve 15.6% power conversion efficiency. The interesting fact in this work is they found that the presence of graphene has reduced the temperature needed in the fabrication process, usually the needed temperature can reach 500° C. and they were able to produce the same material with only 150° C. (Wang, Jacob Tse-Wei, et al. 2013). This work is a great indication that 2D materials in general can increase the stability of perovskite solar cells. Another work done by Shihe Yang et al, where they used graphene quantum dots as an interlayer between the mesoporous layer TiO2 and the perovskite layer. They have done transient absorption measurements and found the increase in power conversion efficiency is related to improvement of electron extraction process. Without the graphene quantum dots the electron extraction was 260-307 ps. After embedding with graphene quantum dots it was 90-106 ps which explains the improvement in power conversion efficiency from 8.81% to 10.15% (Ke, Weijun, et al. 2015).
Remarkable efforts have been made to improve the photovoltaic (PV) conversion efficiencies, which result in great increases in device performance over the past decades for cost-effective PV yet novel systems. However, some performance issues such as device stability, flexibility, limited ability to scale up, has prevent these perovskite solar cells from achieving their market potential. Beyond graphene, another kind of 2D materials has been demonstrated to enhance the performance of DSSC but not been applied yet to the perovskite solar cells such as hexagonal boron nitride (h-BN). For example, Bin Yu has reported a work whereas h-BN has been used as a passive layer in DSSCs, which resulted in a 57% enhancement when compared without h-BN. It was found that h-BN has reduced the electron-hole recombination which improved the efficiency (Shanmugam, Mariyappan, et al. 2013). h-BN has the ability to increase the thermal stability and mechanical stability due to its unique properties as proven in literature. Since, h-BN has not been applied until this moment to increase the stability of the perovskite solar cell. Thus, there is a need for a better composite and method of preparing such composite to create a more effective material for the perovskite solar cell to improve solar cell efficiency.